Glass-ceramics represent a useful class of biomaterials characterized by the presence of one or more crystalline phases within a glassy matrix. Glass-ceramics are typically very strong, exhibit higher strength than glass, have good corrosion resistance, and are either bioinert or capable of reacting with surrounding tissues as a result of being bioactive. Some biomedical ceramics, such as hydroxyapatite and tricalcium phosphate, are even biodegradable. Glass-ceramics have been used as components in a variety of biomedical applications, such as hip implants, dental implants, middle ear implants, and intervertebral spacers. However, while known glass-ceramics can exhibit outstanding strength when loaded in compression, glass-ceramics can fail at low stresses when loaded in tension or bending as a result of having insufficient tensile strength.
Fluorrichterite glass-ceramics represent a useful type of glass-ceramic biomaterials, and have been extensively studied. They are machinable, heat pressable, exhibit high strength and toughness, excellent chemical durability and remarkable resistance to slow crack growth. The fluorrichterite structure is similar to that of mica with basic building blocks consisting of SiO4 tetrahedra assembled in rings of six. However, as opposed to the mica structure, the orientation of these rings alternates, leading to a more difficult cleavage along the basal planes and better mechanical properties. Previous work by the inventors has shown that these glass-ceramics were potential candidates as dental ceramics. See Denry et al., J. Biomed. Mater. Res. (Appl Biomater), 53(4), p. 289-96 (2000), Denry et al., J. Biomed. Mater. Res. (Appl Biomater) 63(2), p. 146-51 (2002), Denry et al., J. Dent. Res., 81, p. 223, Abstract #1692, (2002), and Denry et al., Dent. Mater, 20(3), p. 213-19 (2004).
One of the compositions studied exhibited a dual microstructure consisting of fluorrichterite prismatic crystals and mica platelets that promoted crack deflections and arrest, leading to a glass-ceramic with high flexural strength and high reliability. Denry et al., J. Biomed. Mater. Res (Appl Biomater), 80B(2), p. 454-59 (2007). However, glass-ceramics for use in dental applications preferably have visual characteristics that facilitate the production of aesthetically appealing restorations.
Lanthanum oxides were a potential candidate for modifying glass ceramics to produce aesthetically appealing restorations. It has been shown that additions of lanthanum oxide to silicate glasses results in an increase in elastic modulus and hardness. Makishima et al., J. Am. Ceram. Soc., 61, p. 247-49 (1978). Lanthanum oxide has also been shown to act as a network modifier in lanthanum aluminosilicate glasses, thereby decreasing viscosity at high temperature. (Aronne A, Esposito S, Pernice P. FTIR and DTA study of lanthanum aluminosilicate glasses. Materials Chemistry and Physics 1997; 51(2):163-168). Other studies have investigated the effect of lanthanum oxide on the bioactivity and properties of calcium silicate glasses. See Fresa et al., J. Biomed. Mater. Res., 32, p. 187-92 (1996), Branda et al., J. Thermal Analysis and Calorimetry, 64(3), p. 1017-24 (2001), and Costantini et al., Thermochimica Acta, 372(1-2), p. 67-74 (2001). For example, it was shown that lanthanum can partially substitute for calcium without significantly altering the bioactivity. Accordingly, there is a need to evaluate the effect of lanthanum oxide on the optical properties, crystallization behavior and microstructure of fluorrichterite glass-ceramics, to determine if lanthanum oxide is a suitable doping agent for tailoring the strength and processing conditions of fluorrichterite-based glasses.